Method of separating layers of material

ABSTRACT

A lift off process is used to separate a layer of material from a substrate by irradiating an interface between the layer of material and the substrate. According to one exemplary process, the layer is separated into a plurality of sections corresponding to dies on the substrate and a homogeneous beam spot is shaped to cover an integer number of the sections.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 11/008,589 filed Dec. 9, 2004, pending, which claims thebenefit of co-pending U.S. Provisional Patent Application Ser. No.60/557,450, filed on Mar. 29, 2004, which is fully incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to separation of layers of material andmore particularly, to separation of layers of material, such as asubstrate and a film grown on the substrate, by irradiating an interfacebetween the layers.

BACKGROUND INFORMATION

GaN/InGaN-based Light-Emitting Diodes (LEDs), known as “Blue LEDs,” havea promising future. Practical applications for these GaN/InGaN-basedLEDs have been expanding to include such products as mobile phonekey-pads, LCD backlights, traffic lights, commercial signs, automotivelights, outdoor full-color display panels, household illuminativedevices, and others. In these and other applications, thesehigh-brightness LEDs may replace conventional light sources such asincandescent and fluorescent lights. Blue LEDs are characterized by highlight output at lower energy input than conventional light sources(energy saving, high efficiency) and a longer working life. Their highperformance and reliability shows promise for their successfulreplacement of conventional light sources; however, there is a need toimprove current LED designs to overcome currently-known limitations andinherent drawbacks. Better and more precise manufacturing techniqueshelp advance blue LED design by cutting waste, increasing yields, andallowing more advanced and complex or improved designs to emerge,advancing the technology through more flexibility in Design forManufacturability (DFM). Such improved manufacturing techniques simplifyand reduce the cost of their manufacture.

Blue LED's may be fabricated by depositing GaN/InGaN layer(s) on asapphire substrate. Once the LED devices have been fabricated, the waferis separated into individual dies. One current die separation processinvolves the following steps. First the sapphire wafer is thinned toless than 100 μm in thickness by grinding and lapping the backside ofthe wafer. Next the wafer is mounted to dicing tape and then scribedalong the streets between the die by means of a diamond scribe tip or UVlaser beam. Finally, the wafer is fractured along the scribe lines bymeans of a fracturing tool. After fracturing, the dicing tape isstretched so as to physically separate the die from one another so thatsubsequent automated pick and place operations can be performed. Thisprocess is referred to as “scribe and break” die separation.

A major cost of LED fabrication is the sapphire thinning and thescribe-and-break operation. A process known as LED lift-off candramatically reduce the time and cost of the LED fabrication process.LED lift-off may eliminate wafer scribing by enabling the manufacturerto grow GaN LED film devices on the sapphire wafer, for example, andthen transfer the thin film device to a heat sink electricalinterconnect. In this process, the laser beam profile fires through theback of a sapphire wafer to de-bond the GaN LED device and transfer itto a substrate where it can then be packaged onto a heat sink and/oroptical reflector. Using special wafers, the sapphire growth substratemay possibly be re-used, and the cost of LED fabrication can be reduced.Additionally, this approach is fast, delivering increased LED lightoutput, and has low operating costs due to low stress on the UV laser.

Current designs of GaN LEDs have inherent limitations that hamperefforts to improve performance and reliability. The designs have alsobeen associated with electrostatic discharge problems. As shown in FIGS.1A and 1B, a blue LED 10 may include multiple InGaN and GaN based layers12 a, 12 b, 12 c which are hetero-epitaxially grown on a silicon carbideor a sapphire wafer substrate 14. Since the sapphire wafer is a naturalinsulator, current is supplied by a horizontal electrode configuration.Due to the high resistance of the p-GaN layer 12 a, a thin film of Ni/Au16 is deposited over the p-GaN to promote current dispersion spreading.However, there are some drawbacks associated with the horizontalconfiguration.

First, the Ni/Au film 16 absorbs a substantial portion of the LED lightoutput. The Ni/Au film 16 is very thin (usually less than 100 Å), inorder to make it transparent to LED light, since it has limitedtransmittance to the emitting light. Approximately 25% of the lightemitted by the LED itself is absorbed by the Ni/Au film 16. Furthermore,a significant percentage of the emitted light is lost in transmissionthrough the sapphire. Some of light directed towards the sapphiresubstrate 14 is reflected to the front surface due to the difference inrefractive indices between the sapphire wafer and its surroundings. TheNi/Au thin film 16 absorbs the majority of this reflected output lightas well.

Secondly, the Ni/Au film 16 is sensitive to moisture, resulting inperformance degradation over time. To maintain the film's transparency,thin Ni/Au is deposited by metal evaporation, and then heat-treated inan ambient air or an O₂ environment. The Ni/Au film 16 forms an oxidizedcompound, NiO_(x) with an Au-rich structure. When moisture penetrates tothe oxide film over long-term operation, the LED device 10 will bedamaged.

Third, the Ni/Au film 16 experiences a degradation in the performanceefficiency of the InGaN MQW light-emitting layer 12 b due to a currentcrowding effect. Since the current spreading Ni/Au film 16 has lowerresistance than the n-GaN layer 12 c, the current may crowd in theregion 18 near the n (−) electrode 20 (see FIG. 1A). Thus, thephenomenon of current crowding may prevent homogeneous use of the activeInGaN area, resulting in low efficiency of light output and lowreliability due to uneven use of the active area.

Fourth, the horizontal-electrode configuration may create the effect ofa current bottleneck, resulting in low reliability. The current suppliedthrough the p (+) electrode 22 spreads across the Ni/Au film 16, andflows from p-GaN 12 a through InGaN 12 b to n-GaN 12 c. Since the n (−)electrode 20 is horizontally located at the n-GaN 12 c, the current isbottlenecked in the area 24 at the electrode 20 (FIGS. 1A and 1B).

A LED structured with a vertical electrode configuration overcomes manyof the drawbacks of the horizontal LED structure. As shown in FIG. 2, anLED 30 with a vertical structure involves a transfer of GaN layers 32 a,32 b, 32 c from the sapphire substrate to a conductive substrate 34,such as a silicon wafer. The vertical electrode configuration mayeliminate the Ni/Au film, which substantially increases light output.The vertical structure allows the deposition of a metal reflection layer36, which minimizes light loss through the sapphire in the horizontalstructure. The vertical structure also improves reliability andperformance by reducing or eliminating the current crowding and bottleneck. A factor in constructing the vertical LED structure is thesuccessful lift-off process of the GaN layer from the epitaxial sapphirewafer to the conductive silicon wafer.

One example of the construction of a high-brightness vertical LED isshow in FIG. 3. First, GaN layers 32 a, 32 c are deposited onto asapphire wafer 38. After a metal thin-film reflector 36 is deposited onthe p-GaN, then a Si substrate, or any other conductive substrate 34(including GaAs substrate and thick metal films) is bonded over themetal thin-film reflector. The sapphire wafer is removed by UV-laserlift-off, as described below. The n (−) electrode is deposited on then-GaN layer and the p (+) electrode is deposited on the Si wafer. Sincethe n-GaN layer has lower resistance than the p-GaN layer, the thinNi/Au film is no longer needed. Current is therefore more evenly spreadwithout crowding or a bottleneck effect. Elimination of the troublesomeNi/Au thin film results in an increase in performance and reliability ofLEDs with the vertical structure.

The vertical structure may be created using a UV-laser lift off process.One approach to UV-laser lift-off involves the selective irradiation ofthe GaN/Sapphire interface with a UV laser pulse, utilizing theabsorption difference of UV light between the GaN (high absorption) thinfilm layers and the sapphire substrate. Commonly, the GaN layers arehetero-epitaxially grown on a sapphire wafer. To facilitate GaN crystalgrowth, a buffer layer may be deposited at a relatively low temperature,around 300° C. While the buffer layer helps to grow the GaN layer at ahigh temperature, the buffer layer contains a very high density ofvarious defects due to a large lattice mismatch. The crystal defects,such as dislocations, nanopipes and inversion domains, elevate surfaceenergy which consequently increases absorption of incident UV light. Theincident laser beam for the lift-off process carries an energy densitywell below the absorption threshold of the sapphire wafer, allowing itto transmit through without resulting in any damage. In contrast, thelaser energy density is high enough to cause photo-induced decompositionat the interface, which allows debonding of the interface.

Studies exist regarding the UV laser lift-off process. Kelly et al.demonstrated decomposition of GaN by laser irradiation throughtransparent sapphire, using a Q-switched Nd:YAG laser at 355 nm. (see M.K Kelly, O. Ambacher, B. Dalheimer, G. Groos, R. Dimitrov, H. Angererand M. Stutzmann, Applied Physics Letter, vol. 69p. 1749, 1996). Wong etal. used a 248 nm excimer laser to achieve separation of ˜5 μm thin GaNfilm from a sapphire wafer (see W. S. Wong, T. Sands and N. W. Cheung,Applied Physics Letter, vol. 72 p. 599, 1997). Wong et al. furtherdeveloped the lift-off process on GaN LED using a 248 nm excimer laser(see W. S. Wong, T. Sands, N. W. Cheung, M. Kneissl, D. P. Bour, P. Mei,L. T. Romano and N. M. Johnson, Applied Physics Letters, vol. 75 p.1360, 1999). Kelly et al. also demonstrated the lift-off of 275 μmthick, free-standing GaN film using a raster scanning of Q-switched 355nm Nd:YAG laser (see M. K. Kelly, R. P. Vaudo, V. M. Phanse, L. Gorgens,O. Ambacher and M. Stutzmann, Japanese Journal of Applied Physics, vol.38 p. L217, 1999). Kelly et al. also reported their difficulty inovercoming extensive fracturing of GaN thick film upon the laserlift-off process, due to high residual stresses from a GaN-sapphirewafer. Id. In this study, the authors had to heat the GaN/sapphire waferto 600° C., but they could not completely offset the fracturing problemscaused by the residual stresses.

In spite of the advantages from UV-laser lift-off, GaN LED manufacturinghas been limited due to poor productivity caused by low process yield.The low yield is due in part to high residual stresses in a GaN-sapphirewafer, resulting from a Metal-Organic Chemical Vapor Deposit (MOCVD)process. The MOCVD process requires an activation temperature of over600° C. As shown in FIG. 4A, GaN and InGaN layers 32 are deposited on asapphire wafer 38 by the MOCVD process. Since there is substantialdifference in coefficients of thermal expansion (CTE) between the GaN(5.59×10−6/° K.) and the sapphire (7.50×10−6/° K.) (see Table 1), highlevels of residual stresses exist when the GaN/sapphire wafer cools downto ambient temperature from the high temperature of the MOCVD process,as shown in FIG. 4B. The residual stresses include compressive residualstresses 40 on the GaN and tensional residual stresses 42 on thesapphire.

TABLE 1 Various material properties of GaN and sapphire. Band LatticeLattice Gap Thermal Const. a Const. c Density Energy Expansion MaterialStructure (Å) (Å) (g/cm³) (eV) ×10⁻⁶/° K Sapphire Hexagonal 4.758 12.9913.97 9.9 7.50 GaN Hexagonal 3.189 5.815 6.1 3.3 5.59

When an incident laser pulse with sufficient energy hits a GaN/sapphireinterface, the irradiation results in instantaneous debonding of theinterface. Since the incident laser pulse has limited size (usually farless than 1 cm²), it creates only a small portion of the debonded orlifted-off interface. Since surroundings of the debonded area still havehigh level of residual stress, it creates a concentration of stress atthe bonded/debonded border, resulting in fractures at the border. Thisfracturing, associated with the residual stress, has been one of theobstacles of the UV-laser lift-off process.

Currently, there are different ways to perform laser lift-off processeson GaN/sapphire wafers. One method involves raster scanning of aQ-switched 355 nm Nd:YAG laser (see, e.g., M. K. Kelly, R. P. Vaudo, V.M. Phanse, L. Gorgens, O. Arribacher and M. Stutzmann, Japanese Journalof Applied Physics, vol. 38 p. L217, 1999). This lift-off process usinga solid state laser is illustrated in FIG. 5A. Another method uses a 248nm excimer laser (see, e.g., W. S. Wong, T. Sands, N. W. Cheung, M.Kneissl, D. P. Bour, P. Mei, L. T. Romano and N. M. Johnson, AppliedPhysics Letters, vol. 75 p. 1360, 1999). This lift-off process using anexcimer laser is illustrated in FIG. 5B.

Both processes employ raster scanning, as shown in FIG. 6, whichinvolves either translation of the laser beam 44 or the target of theGaN/sapphire wafer 46. A problem associated with the raster scanningmethod is that it requires overlapping exposures to cover the desiredarea, resulting in multiple exposures 48 for certain locations. In bothof the above methods, the laser lift-off of GaN/sapphire is a singlepulse process. The unnecessary multiple exposures in localized areasincrease the potential for fracturing by inducing excessive stresses onthe film.

As shown in FIG. 7, raster scanning also involves a scanning of thelaser beam 44 from one end to the other, gradually separating theGaN/sapphire interface from one side to the other. This side-to-siderelaxation of residual stresses causes large differences in the stresslevel at the interface 50 between the separated and un-separatedregions, i.e., the interface between the scanned and the un-scannedarea. The disparity in residual stress levels at the interface 50increases the probability of propagation of Mode I and Mode II cracks.Although the illustrations in FIGS. 6 and 7 are based on a process usinga solid state laser, raster scanning of an excimer laser will producesimilar results.

Currently, a common size of sapphire wafers is two-inch diameter, butother sizes (e.g., three-inch and four-inch wafers) are also availablefor the hetero-epitaxial growth of GaN. For a GaN/sapphire wafer, thelevel of residual stresses varies in the wafer, and compressive andtensile residual stresses may exist together. The existence of theresidual stresses may be observed by wafer warping or bowing. When alaser lift-off process relaxes a large area of a continuous GaN/sapphireinterface, as described above, a severe strain gradient may be developedat the border between the debonded and the bonded interface. This straingradient may cause extensive fracturing of the GaN layer.

When a target material is irradiated with an intense laser pulse, ashallow layer of the target material may be instantaneously vaporizedinto the high temperature and high pressure surface plasma. Thisphenomenon is called ablation. The plasma created by the ablationsubsequently expands to surroundings. The expansion of the surfaceplasma may induce shock waves, which transfer impulses to the targetmaterial. The ablation may be confined in between two materials when thelaser is directed through a transparent material placed over the target.During this confined ablation, the plasma trapped at the interface maycreate a larger magnitude of shock waves, enhancing impact pressures.The explosive shock waves from the confined ablation at the GaN/sapphireinterface can cause not only separation of the GaN layer from thesapphire substrate but may also fracture the GaN layer near the laserbeam spot (see, e.g., P. Peyre et. al., Journal of Laser Applications,vol. 8 pp. 135–141, 1996).

Accordingly, there is a need for an improved method of separating GaNthin films from a sapphire wafer by addressing the problems associatedwith residual stress, which lead to low yields due to the fracturing ofseparated film layers. There is also a need for processes that can beextended to any lift-off applications to address one or more of theproblems discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will be better understood byreading the following detailed description, taken together with thedrawings wherein:

FIG. 1A is a schematic diagram illustrating a cross section of aconventional GaN LED with a horizontal electrode configuration.

FIG. 1B is a top view of the GaN LED shown in FIG. 1A.

FIG. 2 is a schematic diagram illustrating a cross section of a GaN LEDwith a vertical electrode configuration.

FIG. 3 is a flow diagram illustrating construction of a GaN LED with avertical electrode configuration.

FIG. 4A is a schematic diagram illustrating a GaN/sapphire wafer duringa MOCVD process.

FIG. 4B is a schematic diagram illustrating formation of residualstresses on a GaN/sapphire wafer after a MOCVD process.

FIG. 5A is a schematic diagram illustrating a conventional method oflaser lift-off on a GaN/sapphire wafer using a Q-switched 355 nm Nd:YAGlaser.

FIG. 5B is a schematic diagram illustrating a conventional method oflaser lift-off on a GaN/sapphire wafer using a 248 nm excimer laser.

FIG. 6 is a schematic diagram illustrating raster scanning of aQ-switched 355 nm Nd:YAG laser on a GaN/sapphire LED wafer and theresulting multiple exposures.

FIG. 7 is a schematic diagram illustrating raster scanning on aGaN/sapphire LED wafer and the resulting stresses, which create a highprobability of Mode I and II cracks at the interface.

FIG. 8 is a schematic diagram of the use of a laser pulse to induce ashock wave for separating layers, consistent with one embodiment of thepresent invention.

FIG. 9 is a schematic diagram illustrating a laser exposed area andcross-section of the separation of the layers, consistent with oneembodiment of the present invention.

FIGS. 10A–10C are schematic diagrams illustrating the effects ofdifferent laser energy densities.

FIG. 11 is a schematic diagram of a wafer illustrating selectiveablation of GaN layers on streets to separate the GaN layers into aplurality of dies, leaving the sapphire wafer intact, consistent withone embodiment of the present invention.

FIG. 12 is a schematic diagram of a beam delivery system illustratingthe projection of a homogeneous beam and representative beam profileshown along the beam path, consistent with another embodiment of thepresent invention.

FIG. 13 is a schematic diagram of a wafer illustrating laser lift-offexposure using a step and repeat process, consistent with a furtherembodiment of the present invention.

FIG. 14 is a photograph of a wafer illustrating a single pulse exposureon a three-by-three LED array using the step and repeat lift-offprocess.

FIG. 15 is a diagram illustrating a laser lift-off process, combiningthe segregation of residual stress and precision step-and-repeat laserbeam exposure, consistent with yet another embodiment of the presentinvention.

FIG. 16 is a photograph of a wafer illustrating selective removal of GaNby a solid state UV laser with a variable astigmatic focal beam spot.

FIG. 17 is a schematic diagram illustrating concentric or helical laserlift-off exposure with a square beam, consistent with a furtherembodiment of the present invention.

FIG. 18 is a schematic diagram illustrating concentric or helical laserlift-off exposure with a circular beam, consistent with a furtherembodiment of the present invention.

FIG. 19 is a schematic diagram illustrating concentric laser lift-offexposure with a variable annular beam, consistent with a furtherembodiment of the present invention.

FIG. 20 is a diagram illustrating a laser lift-off process, consistentwith yet a further embodiment of the present invention.

DETAILED DESCRIPTION

This detailed description describes exemplary embodiments of processesconsistent with the present invention, which address the problemsassociated with existing lift-off processes and increase productivity.Applications of the invention are not limited to the following exemplaryembodiments. Although the exemplary embodiments refer to GaN andsapphire and the GaN/sapphire interface, other types of substrates andlayers of material may be used which are known to those skilled in theart. Also, a sacrificial layer can be provided between the GaN (or otherlayer of material) and the sapphire (or other type of substrate).

Referring to FIG. 8, a laser may be directed through at least one layerof substrate material 102 to at least one target material 104 toseparate the materials 102, 104. In the exemplary embodiment, thesubstrate material 102 is sapphire and the target material 104 isgallium nitride (GaN). The separation of the materials 102, 104 may beachieved by using a laser energy density sufficient to induce a shockwave at the interface 106 of the target material 104 and the substratematerial 102, thereby instantaneously debonding the target material 104from the substrate material 102. The shock wave may be created by theexplosive expansion of plasma 108 at the interface as a result of theincreased density of the ionized vapor sharply elevating the plasmatemperature. The laser energy density may be in a range sufficient toinduce a force F_(a) on the target material 104 that causes separationwithout fracturing. The applied force F_(a) may be represented asfollows:P _(p)(GPa)=C[I _(r)(GW/cm²)]^(1/2)F _(a)(N)=P _(p)(GPa)A _(r)(cm²)where P_(p) is the peak pressure induced by explosive shock waves, C isan efficiency and geometrical factor, I_(r) is the irradiance of theincident laser beam, F_(a) is the applied force and A_(r) is the areaunder irradiation.

When the plasma 108 is expanding, as shown in FIG. 9, the laser exposedarea is acting as a bending arm pivoting at the edge of the laserexposed area. For example, the force (F_(r)) required for rupturing orfracturing may be viewed as a two-point bend test and may be representedas follows:

$F_{r} \propto {\frac{w\; d^{2}}{L}\sigma_{r}}$where d is the thickness of the target material 104, w is the width ofthe applied force or width of the laser pulse, L is the length ofapplied arm or half length of the laser pulse, and σ_(r) is the modulusof rupture or fracture stress of GaN. To increase the force (F_(r)), thewidth w of the laser pulse may be increased and the half length L of thelaser pulse may be decreased, thereby forming a line shaped beam. Theline shaped beam may be scanned across the target material 104 tominimize the bending moment upon irradiation.

At a laser energy density under the ablation threshold of GaN (˜0.3J/cm² at 248 nm), for example, the instantaneous separation of theGaN/sapphire interface 106 may not be successfully achieved, as shown inFIG. 10A. Although decomposition of the GaN can occur under the ablationthreshold, this alone cannot achieve instantaneous separation of theinterface 106, because there is no driving force, i.e. shock waves fromthe expanding plasma, without the ablation. Conversely, applyingoverly-intense laser energy density may create excessive explosivestress wave propagation, which results in cracks and fractures on thetarget material 104 (e.g., the GaN film), as shown in FIG. 10C. When theirradiating laser energy density is optimized, as shown in FIG. 10B, theforce created by the shock wave is sufficient to separate the layers102, 104 at the interface 106 but not enough to induce fracture in thetarget material 104. According to one exemplary embodiment with GaN andsapphire, the optimum range of laser energy density may be between about0.60 J/cm² to 1.5 J/cm².

The parameters of the laser irradiation, such as the wavelength andenergy density, depend on the types of materials being separated. Forexample, the optimum laser energy density for separating GaN fromsapphire is discussed above. A laser wavelength of 248 nm is alsodesirable for separating GaN from sapphire. It is well known to thoseskilled in the art that the photonic energy of 248 nm (5 eV) is betweenthe bandgaps of GaN (3.4 eV) and sapphire (9.9 eV). This indicates thatthe 248 nm radiation is better absorbed in GaN than in sapphire and theselective absorption causes the ablation resulting in separation.

Those skilled in the art will recognize that other laser wavelengths maybe used to separate other types of materials. For example, a bufferlayer may be used between the sapphire substrate and the GaN layer(s) tofacilitate epitaxial growth of the GaN. Examples of the buffer layerinclude a GaN buffer layer and an Aluminum Nitride (AlN) buffer layer.Where an AlN buffer layer is used, a laser at 193 nm may be used becausethe photonic energy of the 193 nm laser light (6.4 eV) is in betweenbandgaps of sapphire (9.9 eV) and AlN (6.1 eV).

According to one embodiment of the present invention, as shown in FIG.11, one or more of the layers to be separated (e.g., the GaN film orlayers) may be formed into smaller areas or sections 112 before lift-offor separation from a substrate 110 such as a sapphire wafer. In oneembodiment, the sections 112 may be segregated, for example, tocorrespond to LED dies. The formation of sections 112 reduces fracturesinduced by residual stresses and shock waves at the interface during alift-off process. The sections 112 of GaN film are less influenced byinduced residual stresses from its surroundings. Furthermore, thesections 112 have an insignificant amount of residual stresses andstrains, which the thin GaN film in these sections 112 can withstand.

According to one example, a GaN/sapphire LED wafer 116 containssymmetric and repeating patterns of sections 112 to form the same-sizedLED dies, which are generally in a few hundreds of microns of square orrectangular size. The symmetrical and repeating sections 112 may beseparated by streets 114, for example, which determine borders for theLED dies and provide sacrificial spaces for die separation, for example,using a scribe and break process. Although the sections 112 in theexemplary embodiment correspond to individual square-shaped dies, thoseskilled in the art will recognize that other configurations and shapes,such as rectangular shapes, may be formed.

The GaN film can be separated into sections 112 through selectivelyremoving or etching of GaN layer(s) on the streets 114. One method ofselectively removing GaN layer(s) on the streets 114 is through reactiveion etching, which is generally known to those skilled in the art. Thisprocess has a few drawbacks, including slow etch rate and the requisitehandling of hazardous chemicals. Another method includes the selectiveetching by a solid state UV laser with a variable astigmatic focal beamspot formed by an anamorphic beam delivery system, as disclosed in U.S.patent application Ser. No. 10/782,741, which is fully incorporatedherein by reference. The variable astigmatic focal beam spot caneffectively adjust its size to an optimum laser energy density, whichselectively ablates GaN layer(s) on the streets 114, leaving thesapphire substrate unaffected (see FIG. 11). This selective GaN ablationutilizes the large difference in ablation threshold between GaN (0.3J/cm² at 248 nm) and sapphire (over 2 J/cm² at 248 nm).

According to another method, the etching can be performed using apatterned laser projection (e.g., using an excimer laser). A patternedexcimer laser beam can also be used to dry pattern the GaN streets orthe devices into shapes or to pattern other thin films such as ITO,metallization, or dielectric insulation layers, or for other devices orconductive or insulator layers. As an alternative to removing portionsof a continuous GaN film to form the sections 112 and streets 114, theGaN can be formed (e.g., grown) on the substrate 110 as sections 112 andstreets 114. The growth of the continuous GaN film, however, may be moreeconomical as compared to the growth of GaN layers with patterns ofstreets 114 and sections 112.

According to a further method, the streets 114 between the sections 112may be widened, for example, using reactive ion etching, after thesubstrate 110 has been removed. Re-etching the streets 114 may reduce oreliminate the possibility of current leakage at the side walls of thesections 112, for example, at the n-GaN and p-GaN junction.

A lift-off process may be used to separate the sections 112 (e.g., theGaN layer(s)) from the substrate 110 (e.g., the sapphire wafer) byirradiating an interface between the substrate 110 and the sections 112.The exemplary laser lift-off process may use a single-pulse process witha homogeneous beam spot and an energy density sufficient to induce ashock wave as described above. The single pulse process avoidsoverlapping exposures at the interface between the substrate 110 and thesections 112 and thus minimizes fracturing. The homogeneous beam spotmay be used to irradiate the interface between layers being separated tosubstantially eliminate the density gradient, thereby facilitatingeffective lift-off. Both UV solid state lasers and excimer lasers can beused with a beam homogenizer to generate a homogenous beam spot for thelift-off process. One exemplary embodiment uses a KrF excimer laser at248 nm. The gaseous laser medium with electrical discharge generateshigh average power with a large raw beam size. The application of thebeam homogenizer is very effective with the large and powerful raw beamof the excimer laser. Also, providing an evenly-distributed laser energydensity in a beam spot advantageously creates effective lift-off in thearea with the single pulse irradiation.

FIG. 12 illustrates one example of a projection of a homogeneous beam bynear-field imaging and shows a representative beam profile along thebeam path. The raw beam from an excimer laser 120 has Gaussiandistribution in short sided/flat topped distribution in the long side.The beam homogenizer 122 (e.g., of multi-array configuration) makes thegradient raw beam profile into a square flat-topped profile. Thehomogenized beam is cropped by the mask 124 (e.g., the rectangularvariable aperture) to utilize the best portion of the beam, which isprojected to the LED target wafer 116 by near-field imaging, forexample, using beam imaging lens 126. The edge resolution of thehomogeneous beam spot 130 at the LED wafer 116 therefore becomes sharp.Although one configuration for the beam delivery system is shown in thisexemplary embodiment, those skilled in the art will recognize that otherconfigurations may be used to create and project the homogeneous beam.Although the exemplary embodiment shows a mask 124 with a rectangularaperture, any shape of mask can be used for near field imaging.

According to one exemplary method, the lift-off exposure is performedusing a step and repeat process. The homogenized beam spot 130 is shapedto include an integer number of segregated sections 112 (e.g.,corresponding to an integer number of LED dies), as shown in FIG. 13.The size of the beam spot 130 is precisely shaped, based on the size ofsegregated sections 112, to include multiple segregated sections 112,such as a three-by-three array. The interface between the substrate andthe integer number of sections 112 may be irradiated using a singlepulse exposure, and the process may be repeated for each group ofsections (i.e., dies). The numbering in FIG. 13 shows an exemplarysequence for the step and repeat process. As the irradiation is repeatedfor each group of sections 112, stitching of the beam spot 130 may beperformed. Advantageously, the stitching may be performed within thestreets 114 to avoid possible damages in the active LED area. In theexemplary process, the stitching of the beam spot 130 is kept withinabout 5 μm

FIG. 14 illustrates an example of a single-pulse exposure on an LEDlift-off wafer by 248 nm excimer laser. In FIG. 14, the homogenized beamspot is covering nine (9) LED dies and the debonded GaN/sapphireinterface appears brighter.

Due to its precisely controlled exposure with a single pulse in a smallarea, the exemplary laser lift-off exposure does not require heating ofthe LED wafer to offset the residual stresses. Exposure may be performedat room temperature. Because the laser light of the lift-off exposuretravels through the sapphire wafer, damage or debris on the surface ofthe sapphire can make shadows at the GaN/sapphire interface, causingdefects on the lift-off interface. The surface of the sapphire may bepolished to remove any debris or particles. The lift-off exposure mayalso be applied to the target in a range of different angles, which willreduce or eliminate shadowing effects.

The exemplary processes discussed above can improve the productivity andyield of the UV-laser lift-off process for successful industrialapplications. An exemplary method consistent with the present inventioncombines the segregation of residual stress on a LED wafer and thehomogenous beam laser exposure. The selective etching of GaN layers onstreets isolates the film into small areas, which have minimal influenceby residual stresses from its surroundings. In addition, the small areasthemselves have minimal residual stresses, which will hardly affect theGaN film upon lift-off exposure. The homogenized beam deliverssubstantially uniform laser energy density in the spot. The preciselaser exposure with a homogenized laser beam allows the proper lift-offwith optimum laser energy density.

FIG. 15 illustrates an exemplary lift-off process. After one or more GaNlayer(s) 132 are grown on a sapphire substrate 110, a protection coating135 can be applied to prevent deposition of laser generated debris onthe GaN layer(s) 132 in the wake of laser scribing. The selectiveremoval of the GaN layer(s) 132 to form streets 114 and sections 112 canbe done by laser scribing or by reactive ion etching. A conductivesubstrate 134 is bonded on the GaN layers 132, after the protectioncoating 135 is removed. The conductive substrate 134 can be any type ofconductive ceramics and metals, including but not limited to, Si, Ge,GaAs, GaP, Copper, Copper Tungsten and Molybdenum. A reflective layer(not shown) may also be formed between the GaN layer(s) 132 and theconductive substrate 134. Then, the sapphire substrate 110 can beremoved by the laser lift-off process. After the laser lift-off, the GaNsurface can be treated for deposition of electrode metal film or othernecessary steps. Finally, the wafer can be separated between thesections 112, for example, to form individual LED dies.

An example of the selective removal of the GaN layer on an actual waferis shown in FIG. 16, where a solid state UV laser providing a high speedlaser cut with a variable astigmatic focal beam spot was used for theGaN removal. In this example, monolithic GaN layers, which contain nodie or street pattern, were grown on a sapphire wafer initially. The LEDdie size is defined by the lines cut by the laser. In the example, thewidth of selective removal or laser cuts is only about 5 μm, whichminimizes the loss of the wafer real estate.

Among the conductive substrates for the lift-off, Molybdenum hasdesirable properties, such as matching coefficient of thermal expansion(CTE), high reflectivity in blue spectra and high strength with lowductility. Molybdenum has a CTE (4.8×10⁻⁶/K) that is relatively close tothat of GaN (5.6×10⁻⁶/K). Metal compounds, such as PdIn and SnAu, may beused for the bonding of the conductive substrates on GaN. When usingthese bonding materials, the GaN and the substrate is heated, forexample, up to around 400 C.°. A large mismatch of CTE between the GaNlayers and the lift-off substrate may introduce another high level ofresidual stresses, which are detrimental to the bonding process. Forexample, although Cu has great thermal and electrical conductivity, itis not as desirable as a lift-off substrate with a 2 inch GaN/sapphirewafer because of its high CTE (16.5×10⁻⁶/K).

Molybdenum has reflectivity of about 55% in the blue spectral region,ranging from 350 nm to 450 nm. This value is comparable to other metals.For instance, the reflectivity values of major metals at 410 nm are asfollows: gold (37%), copper (51%), Nickel (50%), platinum (57%), iron(58%), chromium (70%), silver (86%), aluminum (92%). Although thecomparable reflectivity allows molybdenum to be directly used as areflector (i.e., without a separate reflective layer), the light outputcan be maximized by deposition of a metal film with high reflectivity,such as aluminum and silver. A highly reflective film layer between GaNand molybdenum can increase the performance of a blue LED, for example,without introducing a high level of residual stresses. For example,aluminum can be deposited by sputtering on the GaN surface to form thereflective layer. Since the oxidation of aluminum film is detrimental tothe bonding to the molybdenum substrate, another layer of metallic filmcan be deposited to prevent the oxidation and enhance the bonding.Examples of metallic films that do not oxidize and that will allow themolybdenum to adhere to the aluminum film include, but are not limitedto, tin, zinc, lead and gold.

Molybdenum also provides advantages during the die separation process.Conventional diamond saw or diamond scribing are difficult to use forseparation of a metal film, mainly due to its high ductility. Lasercutting and scribing is an alternative method for die separation.However, a metal film with high ductility, such as copper, requires 100%through cut for the separation, because the mechanical breaking isdifficult on ductile substrates. Thus, the laser through-cut raiseshandling issues because it may not maintain the integrity of small diesafter the cut. Molybdenum has high strength and low ductility. Theseunique mechanical properties of molybdenum facilitate the mechanicalbreaking, even when it is laser-scribed for about 90% of its thickness.

According to another exemplary method, a laser lift-off exposure may becombined with a technique of high speed motion control to maximizeproductivity. When the laser lift-off utilizes the step-and-repeatexposure with precisely designed beam stitching, it is desirable for thetriggering of the laser to be accurate on the target. The fastestpossible speed of the step-and-repeat process is also desirable toincrease productivity. A special function of motion control can be usedto compare the position of the motion stages and send a trigger signalto the laser at predetermined positions. The technique is referred to as‘position compare and trigger’ or ‘fire on fly.’ While motion stages arein continuous motion, a processor in a motion controller is constantlycomparing an encoder counter to user programmed values, and sending outtrigger signals to a laser with matching values. Thus, the motion stagesdo not need to stop for the step and repeat, but may move in continuousmotion, i.e., the laser fires on fly. For example, when the lift-offprocess utilizes a fire on fly technique, the homogeneous beam spot sizeof 1×1 mm², with pulse repetition rate of a laser at 200 Hz, can performthe lift-off process of 2 inch diameter LED wafer within about a minute.

Although the exemplary embodiments involve forming the streets 114 andsections 112 before performing the lift-off process, the techniquesdescribed herein may also be used to separate continuous layers withoutfirst segregating a layer into sections. Although effective separationof continuous layers is possible, there may be micro-cracks formed wherethe laser pulses overlap.

Other exemplary methods may use unique techniques to scan the laser beamfor the lift-off exposure, for example, to irradiate in a concentricpattern. These techniques may be used to perform lift-off of one or moresegregated layers or one or more continuous layers on a substrate. Theresidual stresses in a GaN/sapphire LED wafer have a concentricdistribution, where tension and compression exist together. The laserexposure, when crossing the wafer center, may cause large differences inthe stress level at the interface between the separated and un-separatedregions, i.e., the interface between the scanned and the un-scannedarea. According to different methods, the beam may be scanned with acircular, spiral or helical exposure to relax the residual stressesalong with locations at the same level of stresses. This method reducesthe stress gradient at the interface between scanned and unscannedareas. Alternatively, a line beam with dimensions to minimize bendingmoment upon irradiation may be scanned across the interface, asdiscussed above.

FIG. 17 shows a concentric lift-off exposure with a square beam spot150. FIG. 18 shows a concentric lift-off exposure with a circular beam152. In one method, the laser beam is stationary while the wafer istranslated concentrically (e.g., in a circular or helical pattern) forthe exposure. According to another method, the beam may be moved (e.g.,in a circular or helical pattern) on a stationary wafer.

One way to move the circular beam is using galvanometer scanners, whichprecisely control two mirrors in motion by rotary motors. Other beamspot shapes known to those skilled in the art may also be used, such astriangular, hexagonal or other polygon shapes. In the case of a polygonshaped laser pattern irradiating a polygon shaped die, the beam may bemoved in a circular or spiral motion to overlay the die or groups of dieand provide separation of the film from the substrate in a controlledpattern to relieve stresses in a controlled way.

Another alternative achieves the concentric scanning using a variableannular beam spot. As shown in FIG. 19, the variable annular beam spot154 gradually reduces its diameter to concentrically scan from outeredges to the center of the wafer. The variable annular beam spot can beachieved by an incorporation of two conical optics into the beamdelivery system (BDS), where the distance between the two opticsdetermines the diameter of the spot. Using this annular beam spot movingconcentrically provides stable relaxation of the residual stresses uponthe laser lift-off exposure.

FIG. 20 illustrates a laser lift off process for separating anelectroplated substrate, consistent with a further embodiment. Asapphire wafer or substrate 110 with sections 112 of GaN formed thereonmay be electroplated with a metal or metal alloy to form a metalsubstrate 160. Nickel or copper, or alloys thereof, may be used forelectroplating. The metal substrate 160 may then be cut at locations 162between the sections 112, for example, using a UV laser. A supportingfilm 164 may be mounted on the metal substrate 160, and a laser lift-offprocess such as described above may then be used to separate thesapphire substrate 110. Post laser lift-off processes such as contactmetallization may then be used to remove portions of the metal substrate160 to form dies 166. The dies 166 may then be separated. By cutting themetal substrate 160 prior to laser lift-off, the integrity of the dies166 can be maintained because of the bonding to the sapphire substrate110. Those skilled in the art will recognize that this process may alsobe performed using other materials.

In summary, according to a method consistent with one aspect of thepresent invention, first and second substrates are provided with atleast one layer of material between the substrates, the layer ofmaterial being segregated into a plurality of sections separated bystreets. A beam spot is formed using a laser and shaped to cover aninteger number of the sections. An interface between the first substrateand the sections is irradiated using the beam spot. The irradiating isperformed repeatedly for each integer number of the sections until thefirst substrate is separated from all of the sections.

According to another method, a substrate is provided having at least onelayer of material formed thereon and a homogenous beam spot is formedusing at least a laser and a beam homogenizer. An interface between thelayer and the substrate is irradiated with substantiallyevenly-distributed laser energy density using a single pulse of thehomogeneous beam spot to separate the layer from the substrate.

According to yet another method, a substrate is provided having at leastone layer of material formed thereon and a beam spot is formed using alaser. An interface between the first substrate and the layer isirradiated using the beam spot. The interface is irradiated in agenerally concentric pattern to separate the layer from the substrate.

According to a further method, a first substrate is provided having atleast one layer of material formed thereon and the layer(s) of materialare etched to segregate the layer(s) into a plurality of sectionsseparated by streets on the first substrate. A second substrate isattached to the sections and a homogenous beam spot is formed using alaser. The homogeneous beam spot is shaped to cover an integer number ofthe sections. An interface between the first substrate and the sectionsis irradiated using the homogeneous beam spot. The irradiating isperformed repeatedly for each integer number of the sections. The firstsubstrate is separated from all of the sections.

According to yet another method, a first substrate is provided having atleast one layer of GaN formed thereon and at least one film is formed onthe GaN layer. The film may include a reflective film. A secondsubstrate including Molybdenum is attached to the film and an interfacebetween the first substrate and the GaN layer is irradiated to separatethe first substrate from the layer of GaN.

While the principles of the invention have been described herein, it isto be understood by those skilled in the art that this description ismade only by way of example and not as a limitation as to the scope ofthe invention. Other embodiments are contemplated within the scope ofthe present invention in addition to the exemplary embodiments shown anddescribed herein. Modifications and substitutions by one of ordinaryskill in the art considered to be within the scope of the presentinvention, which is not to be limited except by the following claims.

1. A method of separating at least one layer of material from asubstrate, said method comprising: providing a substrate having at leastone layer of material formed thereon; forming a homogenous beam spotusing at least a laser and a beam homogenizer; and irradiating aninterface between said layer and said substrate with substantiallyevenly-distributed laser energy density using a single pulse of saidhomogeneous beam spot to separate said layer from said substrate,wherein irradiating said interface includes moving said substrate withsaid layer in a generally circular direction.
 2. The method of claim 1wherein forming said homogeneous beam spot includes passing a raw beamthrough said beam homogenizer and then through a variable aperture. 3.The method of claim 2 wherein said variable aperture has a generallyrectangular shape.
 4. The method of claim 1 wherein said laser includesan excimer laser.
 5. The method of claim 1 wherein said laser energydensity of said homogeneous beam spot is sufficient to induce a shockwave at said interface, wherein said shock wave separates said firstsubstrate from said layer of material.
 6. The method of claim 1 whereinsaid layer is segregated into an array of dies on said substrate.
 7. Themethod of claim 1 wherein irradiating said interface includes exposingsaid interface to laser light at a range of angles with respect to saidinterface.
 8. The method of claim 1 wherein said substrate is a sapphirewafer, and wherein said at least one layer of material includes at leastone layer of GaN.
 9. The method of claim 8 wherein said at least onelayer further includes a GaN buffer layer.
 10. The method of claim 9wherein said homogeneous beam spot is formed using a 248 nm excimerlaser.
 11. The method of claim 9 wherein said at least one layer furtherincludes an AlN buffer layer.
 12. The method of claim 11 wherein saidhomogeneous beam spot is formed using a 193 nm excimer laser.
 13. Themethod of claim 1 wherein said laser energy density is in a range ofabout 0.60 J/cm² to 1.6 J/cm².
 14. A method of separating at least onelayer of material from a substrate, said method comprising: providing asubstrate having at least one layer of material formed thereon; forminga beam spot using a laser; and irradiating an interface between saidfirst substrate and said layer using said beam spot, wherein saidinterface is irradiated in a generally concentric pattern to separatesaid layer from said substrate, wherein said beam spot is a variableannular beam spot, and wherein a diameter of said variable annular beamspot is reduced to concentrically scan said interface between said firstsubstrate and said layer.
 15. The method of claim 14 wherein said beamspot is a homogenous beam spot.
 16. The method of claim 14 wherein saidlaser includes an excimer laser.
 17. The method of claim 14 wherein saidlayer is segregated into an away of dies on said substrate.
 18. Themethod of claim 14 wherein said substrate is a sapphire wafer, andwherein said at least one layer of material includes at least one layerof GaN.
 19. The method of claim 18 wherein said at least one layerfurther includes a GaN buffer layer.
 20. The method of claim 19 whereinsaid beam spot is formed using a 248 nm excimer laser.
 21. The method ofclaim 18 wherein said at least one layer further includes an AlN bufferlayer.
 22. The method of claim 21 wherein said beam spot is formed usinga 193 nm excimer laser.
 23. The method of claim 14 wherein saidinterface is irradiated with a laser energy density in a range of about0.60 J/cm² to 1.6 J/cm².